Yet just 36 hours ahead of the self-imposed deadline, U.S. officials are still negotiating on some of the very biggest hurdles that would prevent Iran from using its nuclear technology to build an atomic weapon.

A self-imposed deadline of midnight March 31st may be reconsidered.

The amount of research and development into uranium enrichment that Iran is allowed to conduct in the final five years of the emerging 15-year agreement is a key sticking point.

Ali Vaez, senior Iran analyst for the International Crisis Group, told CBS News that both sides are taking tougher positions on some issues and more flexible positions on others. “This process, at this point, is one step forward, two steps back, or two steps forward and one step back. It is a very complicated diplomatic dance that will go on until the last minute of the last day in these negotiations.”